Context: Ambient particulate matter (PM) is associated with acute exacerbations of airflow obstruction. Additionally, elderly individuals are more susceptible to increased functional morbidity following acute PM exposure. Objective: The purpose of this study is to determine the aging effects of PM exposure on the responsiveness of airway smooth muscle in mice. We hypothesized that airway reactivity induced by methacholine (Mch) will increase with age in PM exposed mice. Materials and methods: Male C57BL/6 (B6) mice at 11, 39, 67, and 96 weeks of age were exposed to carbon black (CB) or room air (RA) for 3 h on 3 consecutive days. One day after the last exposure, mice were anesthetized and airways resistance (R(aw)) was measured by forced oscillation following half-log dose increases of aerosolized Mch. Results: Baseline R(aw) was significantly lower in 67 and 96 week mice compared to 11-week mice (p < 0.05). In RA exposed mice, an age-dependent decline in Mch-induced airway reactivity occurred in association with the highest Mch doses at ages 67 and 96 weeks (p < 0.05). A significantly (p < 0.05) greater Mch-induced R(aw) response occurred in 67-week mice exposed to CB compared with age-matched RA-exposed mice. Discussion and conclusion: Our results show a progressive decrease in the Mch-induced R(aw) response with age in mice. The effect of CB exposure resulted in greater airway reactivity in middle-aged mice, which highlights the effects of PM exposure on the lung as it relates to increased morbidity and mortality with older age.